EP 300 612 A1 discloses aqueous core-shell polymer particles with a crosslinked core and a stabilizing shell with salt groups usable as binders in water-borne base coats useful for the production of base coat/clear coat two-layer coatings. As is clear from the Examples section of EP 300 612 A1, the core-shell polymer particles are produced by initially emulsion polymerizing a mixture of olefinically unsaturated, free-radically polymerizable monomers, which mixture comprises olefinically polyunsaturated monomers, but no acid-functional monomers, and then, in the presence of the resultant emulsion polymer, emulsion polymerizing a monomer mixture which comprises acid-functional monomers but no olefinically polyunsaturated monomers, followed by neutralization of the acid groups.
WO 01/72909 discloses aqueous emulsion polymers usable as binders in water-borne base coats useful for the production of base coat/clear coat two-layer coatings, which emulsion polymers are produced by emulsion polymerization of a first monomer mixture comprising olefinically unsaturated, acid-functional monomers and no olefinically polyunsaturated monomers, neutralization of the acid groups of the emulsion polymer obtained in this manner and subsequent emulsion polymerization of a second monomer mixture containing olefinically polyunsaturated monomers in the presence of the previously produced emulsion polymer.
U.S. Pat. No. 5,403,894 discloses aqueous core-shell polymer dispersions, from which the core-shell polymer particles may be isolated in the form of a redispersible powder suitable for use as a cement modifier. The aqueous core-shell polymer dispersions are produced by emulsion polymerization of a first monomer mixture comprising olefinically polyunsaturated monomers and olefinically unsaturated, acid-functional monomers, neutralization of the acid groups of the emulsion polymer obtained in this manner and subsequent emulsion polymerization of a second monomer mixture in the presence of the previously produced emulsion polymer. The second monomer mixture here contains no olefinically polyunsaturated monomers. It is neither mentioned nor suggested that the aqueous core-shell polymer dispersions may be used as binders in water-borne base coats useful for the production of base coat/clear coat two-layer coatings.
It has now been found that emulsion polymers which are particularly suitable as binders in water-borne base coats useful for the production of base coat/clear coat two-layer coatings may be obtained if said emulsion polymers are produced by multistage emulsion polymerization, olefinically polyunsaturated monomers are copolymerized in all the stages of the emulsion polymerization and olefinically unsaturated monomers with acid groups are copolymerized in the first stage of the emulsion polymerization and if, after completion of the first stage of the emulsion polymerization, the acid groups are neutralized. The use of olefinically polyunsaturated monomers in all the stages of the emulsion polymerization ensures that the polymer products formed in each stage of the emulsion polymerization exhibit a crosslinked structure or even a gel structure.